1. Field of the invention
This invention relates to a slurry filtration device for separation of slurry into a solid portion and filtrate by filtrating the slurry, and a wet lime-gypsum flue-gas desulfurization system with such a slurry filtration device.
2. Description of the related art
One of the examples of the slurry filtration methods used conventionally for separation of slurry into a solid portion and filtrate is the method of contacting flue gas containing sulfur dioxide with an absorption solution having lime or the like as an absorbent that absorbs the sulfur dioxide so as to recover gypsum. The way to concentrate and separate the gypsum slurry in the wet limestone-gypsum flue-gas desulfurization system (referred simply as "flue-gas desulfurization system" below) is explained below.
This type of the flue-gas desulfurization system produces a slurry having about 20-30 wt% gypsum in an absorption tower and the like. The gypsum (fine particles with an average particle size of about 40 .mu.m) in this slurry is separated from the liquid (absorption solution and water having dissolved salts) in the slurry. The former is collected as a byproduct and most of the latter is returned to the absorption tower again.
It is targeted that the water content of the gypsum is kept about 20 wt % or less in the advantage of handling, because the gypsum having a water content of 22 wt % becomes liquid-like after receiving vibration, which makes it difficult to handle the gypsum during loading or bucketing. The gypsum having a water content of 20 wt % or less does not cause this problem.
A conventional flue-gas desulfurization system carries out the concentration of the slurry and dehydration of gypsum combining a slurry concentration tank called a gypsum thickener and a centrifuge. The gypsum thickener is a kind of precipitation tank for separation (gravity separation using the difference of specific gravities) of concentrated slurry and supernatant liquid from the slurry. Incidentally, the specific gravity is 1.0 for water and 2.32 for gypsum. The thickener concentrates the slurry to about 60 wt %. The centrifuge carries out solid-liquid separation of the slurry concentrated by the thickener using a centrifugal force. The water content of the gypsum after the centrifuge is about 5 wt %.
More specifically, the thickener is not a mere tank but comprises a feedwell which feeds the supplied slurry into the tank so as to disperse them quietly, a rake rotating with low speed by which the gypsum precipitated at the bottom of tank is collected in the area for drawing out the slurry, and their driving mechanisms. Each part of the thickener is lined with rubber or resin, or made of corrosion-proof materials such as stainless steel for anti-corrosiveness.
The centrifuge comprises fixed outer pipe, baskets with high rotational speed and their driving mechanisms, filter cloth, and a scraping device. The outer pipe is generally lined with rubber. Although the buckets of stainless steel are used widely, ones with rubber lining may be used depending on the kinds of the liquid to be treated. The stainless wire netting covered with the weave of P.P. (polypropylene) is widely used as the filter cloth material. The scraping device, which scrapes the dehydrated gypsum off the baskets, is made of stainless steel as main material and its blade tip is made of a material with surface-hardening treatment.
The gypsum-slurry treatment system in the conventional art mentioned above has been extremely complicated and expensive. It has required a large area for installation, and brought high equipment cost. Certainly when gypsum with high purity as a byproduct is sought, such a conventional treatment system is needed. When the required quality of the gypsum is not so high, however, the conventional system is of too high grade. Hence, in the of a simple flue-gas desulfurization system whose target is not the recovery of gypsum with high quality, lowering the equipment cost is often most desirable. Further, when the flue-gas desulfurization system is to be added later to an already working power plant, the usable space is often limited and the area required for the installation of the flue-gas desulfurization system has to be cut as much as possible.